This project aims to define how Moraxella catarrhalis (Mcat) senses quorum signals produced by other organisms. Mcat is among are the predominant agents causing otitis media and other opportunistic airway infections, and our data show that Mcat can sense dihydroxypentanedione (DPD) quorum signals produced by Haemophilus influenzae. We hypothesize that Mcat eavesdrops on quorum signal molecules produced by other pathogens, resulting in increased production of the Hag adhesin and concomitant increases in biofilm resistance properties. In order to address this hypothesis we will complete the following Specific Aims: Specific Aim 1: To identify determinants of DPD uptake by Mcat. Specific Aim 2. To define the determinants of DPD response in Mcat. Specific Aim 3. To define importance of quorum signal eavesdropping in Mcat persistence and virulence during experimental otitis media. Otitis media is among the most common pediatric infectious diseases, affecting the majority of children and accounting for billions of dollars in total health care costs per year. It is clear that a significant proportion of these infections are caused by Mcat, for which there is no currently licensed vaccine. Unfortunately, our ability to treat these infections is increasingly limited by the continued emergence of antibiotic-resistant bacterial strains. The results of these studies will provide significant insight into mechanisms for the persistence of Mcat in the polymicrobial infections that represent the majority of cases. Such insights could be pivotal advances in the design of new means for treatment of chronic Mcat infections.